In a PCM system each message channel, serving as an extension of an individual subscriber line, consists of a succession of multibit words (usually of eight bits each) which are interleaved with words of a certain number of other channels within a recurrent frame period. Each word represents a digitized sample of the instantaneous amplitude of a voice signal or the like. The time positions occupied by the words of the several interleaved channels are referred to as time slots, each of them being subdivided into h bit periods which will be referred to hereinafter as phases. There are p time slots per frame in the case here considered, h = 8 and p = 32. In telephone systems or the like, in which each subscriber line has an incoming and an outgoing branch, each subscriber line has an incoming and an outgoing channel allotted to it during communication.
Since intercommunicating subscribers have different time slots assigned to them, the exchange of information between them requires interim storage of words to be transferred from an incoming to an outgoing frame. Obviously, the complexity of the equipment needed for this purpose increases with the number of subscribers served by a given switching station.